This invention relates to a laser driver circuit, suitable for use in a laser printing system and, more particularly, to the implementation of a pulse modulation of the laser current by diverting laser current around the laser in a bypass during interpulse intervals, the invention including also features of parallel bypass switching stages of emitter-coupled logic (ECL) for reduced inductance and increased bandwidth, as well as use of a pulse-width modulated current-switching regulator to produce the main current for energizing the laser in conjunction with a feedback anti-ripple current source for cancellation of current ripple associated with operation of the current-switching regulator.
The use of a laser, in the form of a diode, has found wide use as a source of light in laser printing systems wherein pulses of the light illuminate a photosensistive material of a recording medium. By scanning the light across the medium, and by imparting imaging data to a train of pulses of the light, an image is constructed on the medium. By way of example, the image may be a message composed of alphanumeric characters, or the image may be in a pictorial format constructed of gray scale.
In the construction of a gray-scale image, pixels of dark and light image are imprinted with a sufficiently high density to appear to the human eye as a continuum wherein a high density of dark marks gives the appearance of a relatively dark region of the image while a low density of dark marks gives the appearance of a relatively light portion of the image. By way of example, such an imaging process is most useful for imprinting medical data, such as copies of x-rays, sonograms, and pictorial data from magnetic resonance imaging.
Protection of a high density of pixels on the recording medium is useful, not only in the production of high quality medical images, but also in the production of the printing of high quality alphanumeric characters, particularly in the case of very fine writing. In order to accomplish the small pixels, it is necessary to focus the laser beam to a small point on the medium, which point is to be illuminated by the laser beam, or is to be left blank by a termination of the laser beam. As a practical matter in the construction of such images, it is important to print the images quickly. This requires a rapid scanning of the laser beam relative to the medium, and a capacity for generating the pulses of light at a high rate, the latter requiring a high frequency operating capability to the electric circuitry which energizes the laser with pulses of current. In addition, in order to expose a pixel of the recording medium with sufficient optical energy to produce a mark in a short interval of time, there is a need for increased optical power output from a laser as well as increased power from the circuitry which drives the laser.
A problem arises in that presently available circuitry does not have both adequate power and adequate speed for rapid generation of high intensity pulses of laser light as would be desired for high resolution in the imaging of pictorial data. Attempts to overcome the problem of increased power with present circuit topologies generally entails utilization of larger heat sinks for dissipation of heat produced by the higher power circuits. However, such a requirement for enlarged heat sinks is incompatible with the relatively small spaces available in modern laser printers for replacement of electrical components as well as for the cooling of the components. As an example of the difficulties entailed in use of present laser circuit topologies, a linear feedback amplifier with a power transistor in the feedback loop has been used generally in the construction of circuits for driving pulses of current through a laser to excite the laser to emit light. However, an increase in the size of the power transistor for increased power handling capacity results in a reduction of the switching speed of the transistor. Furthermore, the enlarged power transistor is wasteful of energy due to the large amount of heat dissipated at the transistor, the heat in itself creating a need for additional heat sinking.